1. Field of the Invention
The present invention relates to a method and apparatus for holding a wafer by its edges, and more particularly to a method for holding a wafer by its edges during a spin operation such as a spin dry operation after a wafer scrubbing operation.
2. Background Information
In the fabrication of semiconductor devices, numerous wet clean steps are performed on a substrate such as a semiconductor wafer at various stages throughout the process. In some cases these wet clean steps may comprise scrubbing the wafer with a brush, and may comprise one or more spin rinses wherein the wafer is sprayed with one or more cleaning solutions and/or water while the wafer is spinning. Finally, the wafer is spun dry at a high rate of speed, typically in the 2500-5000 revolution per minute (RPM) range. In many cases, only the front side of the wafer upon which devices are to be formed is cleaned, so that the wafer may be held on its back side by a vacuum chuck. In many other instances, it is desirable to clean both sides of a wafer. For example, semiconductor substrate manufacturers typically scrub both sides at the completion of the slice manufacturing process. Additionally, other processes may require double sided scrubbing. For example, chemical mechanical polishing (CMP), is carried out in a slurry, which is an extremely dirty environment by semiconductor fabrication standards. Therefore, both sides of the wafer must be scrubbed after the CMP operation to remove all contamination prior to subsequent processing. Finally, double sided scrubbing may be used in any application where it is desired to keep the back side of the wafer clean, for example, to reduce contaminants introduced into equipment and/or the process from the back side of the wafer.
In the double sided scrubbing described above, the wafer cannot be held on the back side during the spin dry operation as is done when only the front side of the wafer is cleaned, since the back side of the wafer must be spun dry as well. To provide for spin drying of both the back and front sides, a wafer holder which grips the wafer on its edges has been used in the prior art. Referring to FIG. 1A, an isometric view of a prior art wafer holder is shown. Nest 101 in which the substrate 105 is held is formed by two slide members 106 each having U shaped uprights 107. Wafer holding bumpers 110 are disposed at the top of the upper portion of the uprights as shown. Each of the wafer holding bumpers 110 has a circumferential groove 111 which grip the wafer 105 by its edge. Slide members 106 can move inwardly and outwardly as shown by arrows 115. In particular, the slide members 106 move outwardly to open the nest to release a wafer 105 to some type of conveyor mechanism. Once the nest is in the open position, the next wafer may be placed within the open nest. The slide members 106 then move inwardly to grip this next wafer.
Referring to FIG. 1B, a cross-sectional view of the structure shown in FIG. 1A is illustrated. As shown, slide members 106 comprises slots 120. Cam roller 125 engages both of slots 120. Cam roller 125 is coupled to rod 126 which is disposed within spindle 117. As rod 126 moves in an upward direction, it pushes slide members 120 outwardly to the open position described previously. As rod 126 moves in a downward direction, cam roller 125 pulls slide members 120 in an inwardly or closed position to hold the wafer. Rod 126 is moved up and down by cylinder 130. In operation, cylinder 130 moves cylinder rod 131, connector bar 132, and collar 135 in an up and down direction. Collar 135 is coupled to rod 126 by pin 133. Spring 137, held by stop member 138, puts a downward bias on collar 135.
It will be appreciated that while cylinder 130 is capable of opening and closing the wafer holder of FIG. 1, it cannot be relied on to reliably maintain sufficient force on the wafer to hold it without causing breakage, during all phases of operation. This is because while the wafer is spinning at high speed, a large centrifugal force is generated requiring a correspondingly large inward force or slide members 106 to hold the wafer. Therefore, the far end of the wafer holder, illustrated by portion 156 comprises counterweighting. During the spinning operation, a centrifugal force is created such that the weight at 156 is pulled in an outward direction, thereby pulling the portion of slide members 106 having uprights 107 and wafer holding bumpers 106 in an inward direction to keep the wafer holder in the closed position. This method suffers from the disadvantage that if the counterweight is imprecisely positioned, and/or the weight of the counterweights is different from the specified value, the centrifugal force will vary. Too great a centrifugal force may result in warpage of bendable substrates such as aluminum, and breakage of inflexible substrates such as semiconductor substrates. An insufficient centrifugal force results in the possibility that the wafer will be flung off the holder during the spinning operation. Further, even if the positioning and weight of the counterweights are within specifications, it will readily be appreciated that at the beginning and the end of the spinning operation, when the rotational speed is small, the centrifugal force will be correspondingly small. Thus, the wafers may be flung from the wafer holder at this point. The situation is further complicated by the fact that the forces operating on the wafer holder vary greatly depending on the stage of operation, i.e., whether stopped, accelerating, spinning at high speed, or decelerating.
What is needed is a wafer holder which can be used to hold a wafer in a rotatable spindle without touching the top or bottom surfaces of the wafer. The holder must not release or damage the wafer, must not be speed sensitive, and must not be capable of being opened accidentally.